problem plotting function

I'm facing a problem when plotting the soluation of the following 
differential equation:
q1:2*6/6.6e-3;
tau=33e-6;
eq:'diff(y,x)+y/tau=q1;
ode2(eq,y,x);
sol:ic1(%,x=0,y=-60e-3);
plot2d(rhs(sol),[x,0,3.3e-3],[y,-100e-3,100e-3]);
plot2d: expression evaluates to non-numeric value somewhere in plotting 
range.



I also noticed that using u_O in stead of y gives trouble with the plot2d 
command, is it possible to use another variable name?


thanks in advance
hugo